Cellular radiotelephone systems are commonly employed to provide voice and data communications to a plurality of subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular radiotelephone systems such as designated IS-54B in North America and the pan-European GSM system have been introduced. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993.
FIG. 1 illustrates a conventional terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone system may include one or more radiotelephones 21, communicating with a plurality of cells 36 served by base stations 23 and a mobile telephone switching office (MTSO) 25. Although only three cells 36 are shown in FIG. 1, a typical cellular network may comprise hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 36 generally serve as nodes in the communications system 20, from which links are established between radiotelephones 21 and the MTSO 25, by way of the base stations 23 serving the cells 36. Each cell will have allocated to it one or more dedicated control channels and one or more traffic channels. The control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link 32 may be effected between two mobile stations 21 or between a radiotelephone 21 and a landline telephone user 33. The function of the base station 23 is commonly to handle the radio communication between the cell and the mobile station 21. In this capacity, the base station 23 functions chiefly as a relay station for data and voice signals.
As illustrated in FIG. 2, satellites 110 may be employed to perform similar functions to those performed by base stations in a conventional terrestrial radiotelephone system, for example, in areas where population is sparsely distributed over large areas or where rugged topography tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system typically includes one or more satellites 110 which serve as relays or transponders between one or more earth stations 130 and radiotelephones 21. The satellite communicates with radiotelephones 21 and earth stations 130 over duplex links 170. The earth station may in turn be connected to a public switched telephone network 140, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 150, each serving distinct geographical coverage areas 160 in the system's service region. A satellite 110 and coverage area 160 serve functions similar to that of a base station 23 and cell 36, respectively, in a terrestrial cellular system.
Traditional analog radiotelephone systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels. As a practical matter well-known to those skilled in the art, radiotelephone communication signals, being modulated waveforms, typically are communicated over predetermined frequency bands in a spectrum of carrier frequencies. These discrete frequency bands serve as traffic channels over which cellular radiotelephones communicate with a cell, through the base station or satellite serving the cell. In the United States, for example, Federal authorities have allocated to cellular communications a block of the UHF frequency spectrum further subdivided into pairs of narrow frequency bands, a system designated EIA-553 or IS-19B. Channel pairing results from the frequency duplex arrangement wherein the transmit and receive frequencies in each pair are offset by 45 Mhz. At present there are 832, 30-Khz wide, radio channels allocated to cellular mobile communications in the United States.
The limitations on the number of available frequency bands may present several challenges as the number of subscribers increases. Increasing the number of subscribers in a cellular radiotelephone system may require more efficient utilization of the limited available frequency spectrum in order to provide more total channels while maintaining communications quality. This challenge is heightened because subscribers may not be uniformly distributed among cells in the system. More channels may be needed for particular cells to handle potentially higher local subscriber densities at any given time. For example, a cell in an urban area might conceivably contain hundreds or thousands of subscribers at any one time, easily exhausting the number of frequency bands available in the cell.
For these reasons, conventional cellular systems employ frequency reuse to increase potential channel capacity in each cell and increase spectral efficiency. Frequency reuse involves allocating frequency bands to each cell, with cells employing the same frequencies geographically separated to allow radiotelephones in different cells to simultaneously use the same frequency without interfering with each other. By so doing, many thousands of subscribers may be served by a system of only several hundred frequency bands.
Another technique which may further increase channel capacity and spectral efficiency is time division multiple access (TDMA). A TDMA system may be implemented by subdividing the frequency bands employed in conventional FDMA systems into sequential time slots, as illustrated in FIG. 3. Although communication on frequency bands f.sub.l -f.sub.m typically occur on a common TDMA frame 310 that includes a plurality of time slots t.sub.l -t.sub.n as shown, communications on each frequency band may occur according to a unique TDMA frame, with time slots unique to that band. Examples of systems employing TDMA are the dual analog/digital IS-54B standard employed in the United States, in which each of the original frequency bands of EIA-553 is subdivided into 3 time slots, and the European GSM standard, which divides each of its frequency bands into 8 time slots. In these TDMA systems, each user communicates with the base station using bursts of digital data transmitted during the user's assigned time slots. A channel in a TDMA system typically includes one or more time slots on one or more frequency bands.
Because it generally would be inefficient to permanently assign TDMA time slots to a radiotelephone, typical radiotelephone systems assign time slots on an as needed basis to more efficiently use the limited carrier frequency spectrum available to the system. Therefore, a task in radiotelephone communication is providing a radiotelephone with access to the system, i.e., assigning time slots corresponding to a traffic (voice or data) channel to a radiotelephone when it desires to communicate with another radiotelephone or with a landline telephone or conventional cellular radiotelephone via the PSTN. This task is generally encountered both when a radiotelephone attempts to place a call and when a radiotelephone attempts to respond to a page from another radiotelephone or conventional telephone.
Access to a radiotelephone communication system may be provided in a number of ways. For example, a polling technique may be utilized whereby a central or base station serially polls users, giving each an opportunity to request access in an orderly fashion, without contention. However, serial polling may be impractical for radiotelephone systems because typical radiotelephone systems may have hundreds, if not thousands, of users. Those skilled in the art will appreciate that serially polling this many users may be extremely inefficient, especially when one considers that many of the users may not desire access at all, or may not desire access at the particular moment they are polled.
For this reason, radiotelephone systems typically use random access techniques, whereby a radiotelephone desiring a traffic channel randomly sends an access request to the base or hub station, which the base station acknowledges by establishing a communications channel to the requesting radiotelephone, if available. An example of a random access technique for a TDMA radiotelephone communication system is that used in the GSM system. In the GSM system, a set of Common Control Channels (CCCHs) is shared by radiotelephones in the system and includes one or more Random Access CHannels (RACH).
Radiotelephones typically monitor the status of the RACH to determine whether other radiotelephones are currently requesting access. If a radiotelephone desires access and senses that the RACH is idle, the radiotelephone typically transmits a random access channel signal, typically including the radiotelephone's identification and an identification of the telephone the radiotelephone desires to contact, in what is often referred to as a "RACH burst." As illustrated in FIGS. 4A and 4B, a RACH burst 410 typically contains several fields, including a plurality of guard bits 420, a sequence of synchronization bits 430, and a sequence of information bits 440. The guard bits 420 are used to prevent overlap of communications occurring on adjacent time slots. The synchronization sequence 430 is used by the receiving station to synchronize with the RACH burst, in order to decode the information contained in the information sequence 440. The information sequence 440 may also include a number of sub-fields, for example, a random reference number field 450 which serves as a "tag" for identifying a particular random access request from a particular radiotelephone.
In a GSM system, a RACH is a dedicated TDMA time slot on a carrier frequency band, used by radiotelephones to request access to the communications system. Radiotelephones typically time their RACH bursts to fall within an assigned TDMA time slot for the RACH, for example, by waiting a predetermined period after a transition in a synchronization signal transmitted by the base station and then transmitting the RACH burst. However, because radiotelephones conventionally use a common TDMA time slot for transmitting RACH burst, there is a possibility of collisions between access requests which are transmitted simultaneously or nearly simultaneously by neighboring radiotelephones. To deal with these collisions, the base station may implement some form of contention-resolving protocol. For example, the base station may refuse to acknowledge simultaneous requests, requiring a requesting radiotelephone to reassert its request if it continues to desire access after failing to establish a channel. Contention-resolving protocols may also use a variety of random delays and similar techniques to reduce the likelihood of radiotelephones engaging in repeated collisions subsequent to a first collision. Contention logic used in the European GSM system is described in The GSM System for Mobile Communications published by M. Mouly and M. B. Pautet, 1992, at pages 368-72.
As the complexity of a radiotelephone communication system increases, it may be desirable to provide additional information to a radiotelephone communication system, as part of the RACH message. For example, in a satellite radiotelephone system, as described in FIG. 2, the RACH message may be designed to allow the radiotelephone to transmit the National Country Code (NCC) and the Destination Code for the called number in a radiotelephone originated call. This information may be generally sufficient to efficiently route radiotelephone-originated calls through the radiotelephone system ground stations and network.
However, there may be cases where this information may not be sufficient in order to route the call efficiently. For example, more than one gateway 130 may be provided in a particular country. It may be desirable to route the radiotelephone originated call from the satellite through the gateway that is closest to the radiotelephone. Unfortunately, the RACH message protocol may not provide a field to use in providing this additional information.
Additional information may be provided by redesigning the protocol for the RACH message, to allow encoding of the additional information. Thus, for example, the information content of the RACH message can be increased to include identification of the nearest gateway. Unfortunately, modification of the RACH protocols may require modification of both layer 1 (physical) and layer 3 (signaling) protocols. This may be difficult to implement in radiotelephone communication, in which protocols have already been standardized. Moreover, modification of these protocols may impact the backward compatibility with existing radiotelephones. Finally, if the RACH message is increased in length, additional delay may be produced in obtaining access to the radiotelephone communication system. Accordingly, there is a need to provide additional information within a RACH message, without requiring modification of the RACH message protocol, and without requiring expansion of the RACH slot length.